(T)issues – A note on pigment dispersion and chroma

I’ve been working with powdered pigments for a (brief) while now and as a result, I’m starting to come to grips with the pros and cons of this approach. Overall, it’s lots of fun – in fact, this is so far my favorite part of the color carbon project. But there are challenges also to this part. For instance, I recently ran into an issue of tissues that didn’t look quite as nice and shiny as they should. And that actually has implications that go far below the surface!

I mentioned in a previous blog that I’ve been in touch with Calvin Grier, exchanging some emails and receiving very relevant and helpful comments from him. One of his remarks basically boiled down to: try and make the best tissues you can. Right, sounds simple enough…or not! One thing I ran into when I tried pigment concentrations as high as he apparently uses them, is that the tissues, in particularly the PR122-based magenta ones, had a bit of a coarse, sandpapery texture to them.

PR122 tissue with 1g pigment per 10g gelatin. Note the matte surface sheen: not good!

On a side note: it seems that Calvin is pouring somewhat thinner tissues than I’ve been doing so far. Indeed this is also something that is explicitly recommended for CMY color work in Sandy’s (et al.) book. They recommend a 0.4 mm wet height, whereas mine are probably close to 1.0 mm wet. Calvin mentioned using pigment loadings in the range of 0.2g per 10g of gelatin to a little less than 1 g per 10g of gelatin for the cyan and magenta pigments. That’s significantly higher especially for the magenta pigment, which I had been using at perhaps 1/8 of that concentration. That’s a big difference!

Anyway, I tried a higher pigment loading, using 0.2% PB15:3 and 0.8% w/v PR122 for a glop with 8% w/v gelatin. I dispersed these pigments for a test batch with a small mortar & pestle I picked up at the arts supplies store. It seemed to work alright at first, but I quickly noticed there was a tendency towards (quite severe) pigment clumping. With the paint muller I had been using earlier, it was a lot easier to get a good dispersion – but only with very small quantities of dry pigment.

The dried tissue with these high-concentration pigments turned out to have this coarse surface texture. Well, apart from the yellow ones: those turned out a lot smoother. But I also noticed during preparation of the glops that the yellow PY154 pigment is pretty easy to disperse very nicely, whereas PB15:3 takes more work and PR122 can be pretty challenging to disperse well.

PB15:3 tissue; 0.25g pigment per 10g gelatin. Note the dark specks: this is severe pigment clumping and ultimately responsible for the matte surface sheen.
PY154 tissue (0.5g pigment per 10g gelatin) is much more decent and smooth, although not perfect either

The surface quality of the tissue is actually not the thing that concerns me. It’s something else. But I have to divulge something first. I splurged. On a gift. For me. Oh…yes! Here it is:

X-rite i1 Pro – a gift to me, from me!

Isn’t it grand? Well, really, I resisted getting one of these for quite a while and rest assured, I didn’t get a brand new one. I found a reasonable deal in the 2nd hand market; this is actually an older model, but more than adequate for my purposes. If you hadn’t googled yet: it’s a photospectrometer, and it allows me to quite accurately and quickly (!) measure things like optical density and color of a reflectance sample. It allows me to make pictures like this one:

a* and b* values from Lab measurement of a tissue exposure test: PR122 to the right, PB15:3 bottom left, PY154 towards the top.

What are we looking at? Well, these are the a* and b* measurements of three tissues, each of which exposed without a negative but with increasing amounts of time. These are the tissues I discussed above, the ones with the matte surface textures and poor pigment dispersion. I sensitized these with 0.5ml of a 4% ammonium dichromate solution, for a tissue area of 12x15cm (ca. 4.5×6″). The tissues were exposed as test strips, with the bands representing exposures of 15 seconds, 30s, 60s, 2 minutes, 4m and 6m. I then measured the Lab* values of each strip of each tissue and took the a* and b* values and placed them in the chart above.

Why a* and b*? Well, this gives me insight into (1) color angle and (2) saturation or chroma, and these are of course relevant parameters for color tissues. The color angles should be such that mixing the colors creates the widest possible gamut. Chroma should be as high as possible for each color for the exact same reason. If you remember the color wheel from the previous blog I just linked to, you might notice you could pretty much overlay the chart above on top of that color wheel. As a result, the plotted dots tell us something about what color they represent, by looking at the quadrant they are in: from the center outward to the right/East is pure red, the opposite direction is green, from center to North is yellow and to South is blue. Paper white is dead center, because after all, it’s colorless – and hence it has infinitely low saturation or chroma. And consequently, chroma is therefore simply the distance between the center of the chart and any measurement/dot.

Looking at the Cyan measurements, for instance, we depart from paper white (zero exposure; no density whatsoever), then pass through the 15 second mark at around (-18, -19) and then we pass through the 30 second measurement, 1 minute etc. The diamond shape is the target I was aiming for. For Cyan and Magenta, I took the higher density targets that Grier mentions in an appendix of one of the parts of his calibration e-books. Since I use a different yellow pigment than he does, I relied on a different source (see page 19) for that one.

With this knowledge in the back of my mind, I figured a couple of things:

  • Only for yellow, I actually hit the chroma target – and it took quite a bit of exposure to get there. Around 3.5 minutes or thereabouts.
  • For magenta and cyan, before reaching the desired chroma, an odd thing happened: the chroma actually reduced with increasing exposure and thus density. By the way: in terms of density (or rather, L* value), I easily hit the targets at very short exposure times (25 to 45 seconds or so!), suggesting the pigment loadings of these tissues, at least for magenta and cyan, are really unnecessarily high.
  • There is an interesting change in hue angle depending on exposure. I think I’ve seen something like this in Grier’s illustrations somewhere and it doesn’t bother me too much, but it does suggest that things aren’t always as linear as you’d want or expect them to be.

The lack of chroma is the one that really bothers me here. Evidently, low chroma means a low gamut, and while I might accept a muted palette somewhere later on in the future (if I ever make it that far…), for now, I want to be able to print flashy colors if I like and not be limited to drab pastels.

But where does this lack of chroma come from? The targets I set for cyan and magenta are based on those of Calvin Grier, and he uses the same pigments – form a different source, perhaps. Well, for sure for the PB15:3, as I use Kremer and he discarded that one due to staining issues, which don’t bother me too much.

Well, I think the pigment clumping issue might in fact be the cause. For a previous version of the cyan tissue, I used the muller instead of the mortar and pestle to disperse the pigment, and had no (significant) clumping issues. Test strips from that tissue indeed look slightly ‘fresher’ or more saturated than the ones with the very high pigment loading and clumping issues. They also didn’t have that matte surface sheen. In fact, I did a similar chart for that tissue and the chroma I hit on that one was very, very close to Grier’s target value. So pretty good chroma. I.e., the matte surface in itself isn’t really the problem – it’s the pigment clumping problem, and that appears to have a big impact on chroma.

So back to the muller, than? As I mentioned, that works OK for very small amounts, but what if I want to scale up? So let me confess one more gift to myself: a very basic, entry-level rock tumbler and a tub of 3mm stainless steel pellets (the shop didn’t stock any ceramic media, sadly).

The cheapest, simplest, most cheerful rock tumbler I could find. Boy, does that rubber tumbler cup smell badly of rubber!

Does this help? Well, I don’t know yet, but I’ve been having fun with it already since it arrived yesterday!

Freshly made (tumbled!) pigment dispersions with 3mm stainless pellets used for tumbling in front

I’ve been making some mostly low-concentration dispersions as it’s actually pretty difficult to make small batches of high concentrations with this tumbler. The minimum volume I can make is around 50ml and I stuck to 100ml for the first tests, using only 1 gram of pigment for the colors and testing higher concentrations with cheaper (carbon) pigments. But are they any good? Well, time will tell, but the first signs are promising:

A quick pigment dispersion test: a couple of drops in a jar of water already tells a story

This is a pretty easy test to do: just put a few drops of the dispersion into a beaker of water and look what happens. I did this here with the 1% w/v PB15:3 and PR122 dispersions and they cloud out quite nicely. The PR122 dispersion is actually so light that some of it just won’t drop to the bottom. More importantly, there are no signs of larger clusters. This is after just one hour of tumbling, so all that’s really happening is decent wetting of the pigment particles. This isn’t actually grinding the pigment down or anything; I don’t think that actually works at all with steel balls as a tumbling media – and I’m skeptical it happens at all with the kind of ball mill I’ve got. But that doesn’t bother me: as long as I can rely on a decent dispersion / wetting action, I’m OK relying on the actual grinding by the pigment manufacturer.

Just for fun, here’s another comparison, of two black pigments. One is a Pbk9 ‘bone black’ or ‘ivory black’, which happens to be a fairly coarsely ground pigment in the product I got. The other is a much more finely divided Pbk7 ‘lamp black’, a classic carbon printing pigment (perhaps THE classic pigment, in fact). Look at this:

A somewhat coarse ivory black Pbk9 on the left, a much finer lamp black Pbk7 on the right.

The photo was taken a few seconds after dropping in the Pbk7 on the right, and I did the Pbk9 immediately before that. The Pbk9 dispersion dropped pretty much instantly to the bottom and shows low tinting strength. Sure, it’s only a 5% w/v dispersion while the Pbk7 is a 10% w/v, but that doesn’t explain the difference by far. Both dispersions were made in the same way, using the tumbler, so the difference is really down to how fine or coarse the pigment itself is. Pbk7 is a very fine pigment in general due to the way it’s made and Pbk9 needs to be ground down, so it will probably always be a little coarser.

What does this mean in practice? Well, I struggle to get a decent black with the Pbk9 pigment if I use it in a concentration of 1.25g per 10g of gelatin in a carbon tissue – that’s dry pigment weight! I just poured some tissues with the Pbk7 dispersion, using only around 0.3g dry pigment per 10g of gelatin and these tissues are jet black, super dense black holes!

So, pigment dispersion really is important. It’s also a vast field of applied science that I know relatively little of. Luckily, I don’t have to make super stable dispersions – it’s perfectly OK if I need to shake up a jar of pigment dispersion before dosing it into a batch of glop, so I don’t need fancy stabilizers etc. But I feel the little tumbler was probably a good idea. Now let’s hope it helps to get out the consistently high chroma that the pigments have in them.

And there’s much more! But that’s for another blog. Stay tuned!

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